Mobile telephony networks (Public Land Mobile Networks, PLMNs) were initially conceived for enabling voice communications, similarly to the wired networks (Public Switched Telephone Networks, PSTNs), but between mobile users. Mobile telephony networks have experienced an enormous spread, especially after the introduction of second-generation mobile cellular networks, and particularly digital mobile cellular networks such as those complying with the Global System for Mobile communications (GSM) standard (and its United States and Japanese corresponding systems).
The services offered by these cellular networks in addition to plain voice communications have rapidly increased in number and quality; just to cite a few examples, Short Message Service (SMS) and Multimedia Message Service (MMS) services, and Internet connectivity services have been made available in the last few years. Third-generation mobile systems (e.g. Universal Mobile Telecommunications System, UMTS) are being now deployed and will allow to still increase the offering of services to the mobile users.
Some solutions have been proposed to overcome the limitations of conventional, switched-circuit cellular networks such as the GSM networks, so as to enable users of mobile terminals efficiently exploiting packet-based services, e.g. services offered through the Internet. One of the solutions that is acquiring a significant popularity is the General Packet Radio Service (shortly, GPRS). The GPRS is a digital mobile phone technology compatible with GSM networks (actually, built on the existing GSM network architecture) that enables data transfer at a speed higher than that allowed by pure GSM. Essentially, the GPRS can be viewed as a GSM add-up that supports and enables packet-based data communication. However, third-generation wireless communications systems, such as those complying with the UMTS, are more promising in terms of capability to offer high data transfer rates and guarantee quality of service.
In current GPRS or UMTS communications networks the information content is usually transferred in a point-to-point (p-t-p) modality (or unicast modality), upon activation of a session between a GPRS/UMTS mobile phone (or mobile station) and a service provider connected to a packet data network, e.g. a server connected to the core network or to the Internet; the activation of such a session involves the setting up of logical and physical connections between the server and the GPRS/UMTS mobile phone. In such a p-t-p communication mode, the radio resources to be allocated for the exchange of data between the GPRS/UMTS network and the GPRS/UMTS mobile stations depend on the number of different mobile stations simultaneously exploiting the GPRS/UMTS services, even if two or more mobile station users take advantage of the same information content at the same time. This limits the possibility of simultaneously accessing available GPRS/UMTS services by several users, unless the radio resources are oversized.
Thus, it is desirable to have the possibility of delivering information contents related to a same service exploitable by two or more users at a time through a point-to-multipoint (p-t-m) modality, so as to save the amount of allocated resources.
In this respect, the 3GPP (3rd Generation Partnership Project) is discussing the implementation, both in the GERAN (GSM/EDGE Radio Access Network) and in the UTRAN (UMTS Terrestrial Radio Access Network) frameworks, of a new kind of service architecture, named MBMS (Multimedia Broadcast/Multicast Service). Basically, MBMS targets simultaneous distribution of information content (e.g. multimedia content) to more than one mobile user from a single serving base station over a common radio resource.
Technical Specification 3GPP TS 25.346 V.6.0.0 (2004-03), for example, relates to the introduction of the MBMS services in the Radio Access Network (RAN) of a UMTS network, i.e. in the UTRAN. The document describes techniques for optimized transmission of MBMS bearer service in UTRAN, such as point-to-multipoint (p-t-m) transmission, selective combining and transmission mode selection between point-to-multipoint and point-to-point (p-t-p) bearer. For each MBMS service, data is transferred via an MBMS bearer between the SGSN and the UE (User Equipment). More in detail, for each MBMS service, data is transferred via one MBMS Iu bearer between the SGSN and the RNC (Radio Network Controller) in the whole MBMS service area. One MBMS Iu bearer is established per MBMS service at the MBMS Session Start or when the RNC needs to send data on the radio interface due to the presence of UEs. The MBMS Iu bearer on Iu is established per MBMS service and not per UE individually. Several MBMS RBs (Radio Bearers) may be linked to one MBMS Iu bearer, i.e. one MBMS Iu bearer on Iu may be mapped to multiple p-t-p and/or p-t-m traffic channels over the radio interface. In particular, the MBMS control function in the CRNC (Controlling RNC, i.e. the RNC controlling the MBMS service area) may decide to establish a p-t-m connection if the number of counted MBMS users in a cell exceeds a certain operator-defined threshold. Furthermore, the MBMS control function in the CRNC may decide to establish a p-t-m connection depending on the congestion scenario expected for a specific cell (e.g. in hotspot areas where no bearer type switching is needed). The MBMS control function in the CRNC establishes an MBMS RB by sending service specific signaling messages to all the UEs in the cell listening MBMS point-to-multipoint control channel (MCCH). UEs with activated service(s) may then execute the RB set-up. MBMS data is transferred on a MBMS point-to-multipoint traffic channel (MTCH) to all the UEs which have executed the RB setup. P-t-p transmission of MBMS data should use the DTCH (Dedicated Traffic CHannel) as defined for other dedicated services. According to the present version of the standard, the UE MBMS capability is not sent to the UTRAN. A consequence is that a UE may be counted although its actual capability does not allow to receive MBMS transmissions, e.g. because of its current RRC state. The standard describes a minimum UE capability requirement in order to allow operators to configure MBMS channels that can be common to all UEs supporting the given service.
However, in the document R1-04-0593, submitted by NTT DoCoMo at the TSG-RAN Working Group 1 #37 meeting (Montreal, May 10-14, 2004), the possibility of adopting a two-phase approach for the MBMS bit rate to be supported by MBMS UEs was proposed (Phase 1: 64 kbps or 128 kpbs and less when selective combining is performed, 128 kbps or 256 kpbs and more when selective combining is not performed; Phase 2: 256 kbps and less at all cases), to help the early introduction of MBMS UEs into the market, and assure higher quality for MBMS services later on.